The use of supercritical fluids in industrial processes has been growing at an ever-quickening pace. Replacing traditional, often hazardous and flammable, organic solvents with supercritical fluids has been a prime area of research. Carbon dioxide is a popular choice due to the fact that it is nontoxic, nonflammable, and inexpensive. An attractive feature of a supercritical fluid is that its density can be varied simply by changing the pressure or temperature. Therefore, all density-dependent properties, such as dielectric constant and solubility parameter, can be manipulated in this manner. These key features of supercritical fluids make them ideal candidates for use in extraction and chromatography applications.
In the chemical and pharmaceutical industries, the demand for purified compounds is increasing steadily. It has become highly desirable to obtain components of the highest available purity in the largest quantities. In many instances, high performance liquid chromatography (HPLC) has been the analytical method of choice for these types of separations. HPLC can be analytical or preparative in nature with the component levels varying depending on the application. In the case of preparative HPLC, a collection means is also employed for the sample fractions. However, a drawback to the use of HPLC is the fact that in many instances long elution times, as well as large of amounts of solvents are required for the process.
Supercritical fluid chromatography (SFC) was introduced in the 1980's as an alternative to HPLC. The technique employs a supercritical fluid, typically carbon dioxide, as the mobile phase. In many instances, an organic solvent is also present as a modifier in order to adjust the polarity of the mobile phase. Because supercritical fluids are known for their high diffusivities, this results in enhanced speeds and resolving power when compared to HPLC. The difference can be as much as an order of magnitude in some applications. Additionally, SFC systems can re-equilibrate faster than HPLC systems and therefore can be ready to process other samples in a shorter time frame. Many of the advantages of SFC over HPLC are applicable to both analytical and preparative methods. However, much like HPLC, SFC also needs a means to collect the sample fractions, preferably multiple fractions, in an efficient, cost-effective manner.
Conventional collection systems and methods for SFC have been explored in detail. For example, U.S. Pat. No. 6,413,428 to Berger et al. and European Patent Application No. 117057 to Berger et al., each incorporated by reference herein, disclose a sample collection process for preparative SFC using a collection chamber comprising test tubes. Sample collection methods and systems for SFC are also disclosed in U.S. Pat. No. 5,601,707 to Clay et al., U.S. Pat. No. 6,086,767 to Walters et al., U.S. Pat. No. 6,309,541 to Maiefski et al., and U.S. Pat. No. 5,614,089 to Allington et al., each incorporated by reference herein. While there are several mechanisms for analyte collection in SFC disclosed in the aforementioned patents, the collection systems and methods disclosed therein inefficiently collect fractions, are typically large, complex, difficult to operate, expensive, do not operate at room temperature and standard pressure, typically have a small number of collection tubes, and often require a chemical fume hood.